Gas process or product streams often contain unwanted nitrogen oxides (NOX), the removal of which is desired. For example, combustion waste gases, generally contain large concentrations of carbon dioxide and nitrogen, usually also contain nitrogen oxides due to the slight high temperature oxidation of nitrogen during the combustion process. If it is desired to produce high purity gaseous or liquid carbon dioxide from the waste gas the nitrogen, oxygen, methane, argon and nitrogen oxides must be removed from the waste gas stream. Nitrogen, oxygen, methane and argon can be easily removed from the waste gas by, for instance, distillation, and discharged into the atmosphere, if desired, but the NOX is not easily separated from the carbon dioxide by physical gas separation techniques, nor can it be safely disposed into the environment.
An environmentally acceptable method of removing the NOX from the gas stream is to convert the NOX to nitrogen and then remove the nitrogen from the gas by distillation. This can be accomplished by passing the gas stream over a catalyst at elevated temperatures in the presence of ammonia. The nitrogen oxides are reduced to nitrogen and water vapor, which are subsequently separated from the carbon dioxide. The reaction is highly exothermic; accordingly, if the waste gas contains significant concentrations of NOX, some means of preventing overheating of the catalyst must be employed. This is necessary to prevent inactivation of the catalyst, which occurs at excessively high temperatures. Traditionally, control of the catalyst bed temperature has been accomplished by injecting a stream of air into the gas feed to the NOX reduction reactor. This is not desirable however, when it is desired to recover gas components from the gas stream, because of the need to separate the added air from the desired product.
Because of the importance of removing NOX from gas streams when it is desired to recover certain components of the gas stream, improvements to suitable gas recovery methods are constantly sought. The present invention presents a significant improvement to catalytic NOX reduction operations by providing an efficient solution to the problem of temperature control in the NOX reduction reactor. According to the invention, a portion of the product gas stream exiting the NOX reduction reactor is recycled to the reactor feed.
U.S. Pat. No. 4,718,361 discloses a process in which a fuel is combusted in a furnace with oxygen to produce a carbon dioxide-rich gas. A portion of the carbon dioxide-rich gas is recycled to the feed for mixing with the oxygen, and a portion of cooled carbon dioxide-rich gas is mixed with the combustion gas exiting the furnace to cool the gas.